【正文】 eeing the operator to do other tasks. This gives the CNC user several side benefits including reduced operator fatigue, fewer mistakes caused by human error, and consistent and predictable machining time for each workpiece. Since the machine will be running under program control, the skill level required of the CNC operator (related to basic machining practice) is also reduced as pared to a machinist producing workpieces with conventional machine tools. The second major benefit of CNC technology is consistent and accurate workpieces. Today39。s CNC machines boast almost unbelievable accuracy and repeatability specifications. This means that once a program is verified, two, ten, or one thousand identical workpieces can be easily produced with precision and consistency. A third benefit offered by most forms of CNC machine tools is flexibility. Since these machines are run from programs, running a different workpiece is almost as easy as loading a different program. Once a program has been verified and executed for one production run, it can be easily recalled the next time the workpiece is to be run. This leads to yet another benefit, fast change over. Since these machines are very easy to set up and run, and since programs can be easily loaded, they allow very short setup time. This is imperative with today39。s current position, not from program zero. With this method of manding motion, the programmer must always be asking How far should I move the tool? While there are times when the incremental mode can be very helpful, generally speaking, this is the more cumbersome and difficult method of specifying motion and beginners should concentrate on using the absolute mode. Be careful when making motion mands. Beginners have the tendency to think incrementally. If working in the absolute mode (as beginners should), the programmer should always be asking To what position should the tool be moved? This position is relative to program zero, NOT from the tools current position. Aside from making it very easy to determine the current position for any mand, another benefit of working in the absolute mode has to do with mistakes made during motion mands. In the absolute mode, if a motion mistake is made in one mand of the program, only one movement will be incorrect. On the other hand, if a mistake is made during incremental movements, all motions from the point of the mistake will also be incorrect. Assigning program zero Keep in mind that the CNC control must be told the location of the program zero point by one means or another. How this is done varies dramatically from one CNC machine and control to another8. One (older) method is to assign program zero in the program. With this method, the programmer tells the control how far it is from the program zero point to the starting position of the machine. This is monly done with a G92 (or G50) mand at least at the beginning of the program and possibly at the beginning of each tool. Another, newer and better way to assign program zero is through some form of offset. Refer to . Commonly machining center control manufacturers call offsets used to assign program zero fixture offsets. Turning center manufacturers monly call offsets used to assign program zero for each tool geometry offsets. 畢 業(yè)設計（外文翻譯 ） 5 Fig. 4 Flexible manufacturing cells A flexible manufacturing cell (FMC) can be considered as a flexible manufacturing subsystem. The following differences exist between the FMC and the FMS: 1. An FMC is not under the direct control of the central puter. Instead, instructions from the central puter are passed to the cell controller. 2. The cell is limited in the number of part families it can manufacture. The following elements are normally found in an FMC: ? Cell controller ? Programmable logic controller (PLC) ? More than one machine tool ? A materials handling device (robot or pallet) The FMC executes fixed machining operations with parts flowing sequentially between operations. High speed machining 畢 業(yè)設計（外文翻譯 ） 6 The term High Speed Machining (HSM) monly refers to end milling at high rotational speeds and high surface feeds. For instance, the routing of pockets in aluminum airframe sections with a very high material removal rate1. Over the past 60 years, HSM has been applied to a wide range of metallic and nonmetallic workpiece materials, including the production of ponents with specific surface topography requirements and machining of materials with hardness of 50 HRC and above. With most steel ponents hardened to approximately 3242 HRC, machining options currently include: Rough machining and semifinishing of the material in its soft (annealed) condition heat treatment to achieve the final required hardness = 63 HRC machining of electrodes and Electrical Discharge Machining (EDM) of specific parts of dies and moulds (specifically small radii and deep cavities with limited accessibility for metal cutting tools) finishing and superfinishing of cylindrical/f